Electric discharge tube oscillator



Feb. M, 1939. .A. c. sTocKER 2,147,1 14

ELECTRIC DISCHARGE TUBE OSCILLATOR Filed June 9, 1954 a Sheets-sheet 1 Feb. 14, 1939. A. c. s-rocKER ELECTRIC DISCHARGE TUBE osCILLAToR Fild June 9, 1954 3 Shee 11s-Sheet 2 MJ a TIME

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Feb. 14, 1939. Afc, STOCKER 2,147,114

ELECTRIC DISCHARGE TUBE OSCILLA'I'QR Patented Feb. 14, 1939 UNITED STATES PATENT OFFICE Arthur C. Stocker, Haddon Heights, N. J., assignor to Radio Corporation of America, a, corporation of Delaware Application June 9, 1934, Serial No. 729,748

7 Claims.

My invention relates to electric discharge tube oscillators and particularly to oscillators designed to produce voltage impulses periodically for use in television circuits.

In television apparatus of the type utilizing a cathode-ray tube for transmitting or receiving, or for both, means must beprovided for deilecting the cathode ray both horizontally and vertically simultaneously in order tcscan the view being transmitted and/or the fluorescent screen at the receiver. To obtain undistorted transmission of the view, it is essential that the scanning at the transmitter and at the receiver be maintained in synchronism.

In television systems of the above mentioned type, each cathode-ray tube requires a horizontal deecting circuit and a. vertical deflecting circuit for deflecting the electron beam in the manner described. Each deflecting circuit must supply either voltage or current having a sawtooth wave shape depending upon whether deflecting plates or deflecting coils are employed. It has been the general practice to generate the saw-tooth wave of voltage or-current by generating voltage impulses as by means of the dynatron oscillator and impressing these voltage impulses upon several amplifier tubes connected in cascade for reshaping and amplifying the voltage impulses.

One of the objects of my invention is 4to provide an improved generator of current or voltage impulses.

More specifically, it is an object of my invention to provide-a voltage impulse generator which is stable in operation and which may be readily synchronized.

A further object of my invention is to provide an improved single tube deflecting circuit for a cathode-ray tube.

In a preferred embodiment of my invention, I utilize a screen grid vacuum. tube which has a positive direct-current voltage applied to the screen grid, lthis voltage being somewhat lower than the direct-current voltage applied to the plate as is customary in operating screen grid tubes.

'The plate circuit of the tube is made highly inductive whereby an increase in plate current is almost directly proportional to time.

The plate circuit of the tube is coupled tothe screen grid through a transformer in such phase that the voltage impulsesI in the vplate circuit which make the plate more positive, induces a voltage impulse into the screen grid circuit to make the screen grid negative and thereby bias the tube to plate current cut-off. This is the equivalent of a sudden break in the plate circuit and the resulting collapse of flux in the plate circuit inductance coil induces an exceedingly high voltage in the plate circuit.

(Cl. Z50-36) The above mentioned voltage impulse in the plate circuit occurs after the plate current has increased to a certain value because of the negative resistance characteristic of the screen grid tube.

Other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawings, in which Figure 1 is a circuit diagram of one embodiment of my invention; y N

Figs. 2a, 2b and 2c are curves showing the plate current and certain voltages appearing in the oscillator circuit shown in Fig. 1;

Fig. 3 is a characteristic curve of a screen grid tube;

Figs. 4, 5, 6 and 7 arecurves which are referred to in explaining the operation of the oscillator shown in Fig. 1;

Fig. 8 is a pair of curves showing certain frequency characteristics of my improved oscillator; Figs. 9 and 10 are curves which are referred to in explaining certain characteristics of` my improved oscillator; and

Figs. 1l, 12, 13 and 14 are circuit diagrams showing other embodiments of my invention.

Referring to Fig. l, I have shown one embodiment of my invention applied to a television re` ceiver circuit in'which a cathode-ray receiver tube, indicated at I, is provided with deflecting plates 3 for deflecting the cathode ray horizontally. It Will be understood that, in this case, a voltage wave having a saw-tooth shape is required across the deilecting plates 3.

The oscillator of the saw-tooth Wave generator comprises a vacuum tube 5 having a cathode I which preferably is indirectly heated, a control grid 9, a screen grid I I and an anode or plate I3. The anode I3 is connected to a point of positive potential such as a positive terminal on a battery I5 through thev primary Winding Il of a transformer I9. The screen grid II is connected to a point of positive potential such as a point on the battery I5 through the secondary winding 2I of the transformer I 9 whereby the screen grid circuit is coupled to the plate circuit.

The positive direct-current voltage applied to the screen grid II bythe battery I5 or other voltage supply source is ordinarily considerably lower in value than the positive direct-current voltage applied to the plate I3. For example, if the vacuum tube 5 is type 224, the voltage applied to the anode I3 by the battery may be 500 volts, while that applied to the screen grid Il by the battery may be 90 volts.

In accordance with my invention, the plate circuit of the tube 5 is made highly inductive, this being accomplished in the embodiment being described, by giving the primary winding of the transformer I9 a high value of inductance.

In order to produce the most satisfactory results, the inductance in the plate circuit should be high compared with the total resistance in the plate circuit.

In the circuit illustrated, the horizontal synchronizing impulses, which are transmitted from the television transmitter, are impressed upon the control grid 9 of the tube 5 through a coupling condenser 23. The control grid 9 may be connected to the cathode 1 through a grid leak resistor 25, as shown, or it may be connected to the cathode through a grid leak resistor and a biasing battery (not shown) for applying a negative bias thereto.

To explain in general the operation of the oscillator, it will be assumed that voltage has just been applied to the plate I3. Since the plate circuit is almost purely inductive, the increase in plate current will be almost directly proportional to time, as indicated by the dotted line curve 21 in Fig. 2a. When the plate current reaches a certain definite value, a Voltage lmpulse is produced in the plate circuit in the direction which makes the plate more positive. The reason for this sudden increase in plate voltage will be explained hereinafter.

This voltage impulse in the plate circuit is impressed upon the screen grid Il through the transformer I9, the secondary 2| of the transformer being so connected to the screen grid that the voltage impulse makes the screen grid less positive. As a result, the plate impedance of the tube is increased to cause a decrease in plate current whereby the screen grid is made still less positive, or more negative, and the tube is almost instantaneously brought to cut-off by Van accumulatve action. The result is that the sudden eifective opening of the plate circuit causes a suddent collapse of ux in the primary winding I1 to produce a voltage impulse of great magnitude in the plate circuit, as shown by the curve 29 in Fig. 2b, this voltage impulse appearing in the screen grid circuit as a negative voltage impulse as shown by the curve 3| in Fig. 2c. It will be noted that these negative impulses have a value exceeding the cut-01T voltage of the tube.

Before explaining the operation of the oscillator in more detail, the deecting circuit shown in Fig. 1 will be described. The voltage impulses appearing in the oscillator plate circuit are irnpressed through a coupling condenser 33 upon the input circuit ofv an amplifier tube 35. The plate 31 of tube 35 is connected through a resistor 39 to a source of positive potential such as a point on the battery l or a point on a voltage divider .(not shown). In order to convert the voltage impulses 29 into a voltage having a saw-tooth wave shape, a condenser 4I is connected between the plate 3T and ground whereby it is charged through the resistor 39. As the condenser is charged, the voltage across it increases in a straight line relation Vas indicated by the portion a of the voltage curve 43. As soon as the positive voltage impulse appears on the grid of the tube 35, the condenser 4I is caused to discharge through the tube 35 in a very short time, this period of discharge being represented by the portion b of the voltage curve 43. It will be apparent that the voltage appearing across the condenser 4I has a sawtooth wave shape. The saw-tooth voltage is ampliiied by ampliers 45 and 41 connected in cascade whereby a saw-tooth Wave of voltage of the desired Value is impressed across the primary section of the auto-transformer 49 and, through coupling condensers 5I, upon the deflecting plates 3.

The theory of operation of my improved oscillator will now be described more in detail in connection with Figs. 3, 4 and 5. The curve 53 shown in Fig. 3 is the characteristic plate current-plate voltage curve of a screen grid tube, type 224, for example, for the condition where the control grid and screen grid voltages are held constant. It Will be noted that as the plate voltage increases, the plate current increases up to a certain value and then decreases with further increase of plate voltage until it reaches a certain minimum value where it again begins to increase. The reason for this decrease in plate current, as is well known, is that secondary electrons are emitted from the plate and flow to the screen grid which, at this time, is more positive than the plate.

In explaining the operation of my oscillator, it is preferredto utilize the characteristic curve 53 to see what happens to the plate voltage as the plate current increases. As previously explained, the increase in plate current, because of the inductive plate circuit, will be proportional to time. Therefore, the plate current axis is a time axis as well, as indicated in Fig. 4.

Referring to Fig. 4, it will be evident that at a time when the plate current reaches the value at which it would normally decrease with further increase of plate voltage, the plate voltage suddenly increases to the value indicated by the point 55 on the curve 53. The physical reason for this sudden increase in plate voltage is that, because of the emission of secondary electrons from the plate, there is a tendency for the plate current to decrease, which tendency is opposed by collapsing lines of flux in the primary winding I1. In other words, the inductance in the plate crcuit prevents a sudden decrease in plate current and it does this by causing an increase in voltage across the primary winding of the transformer I9.

Although the upper portion of the characteristic curve 53 of the "screen grid tube has been shown in Fig. 4, the tube operates along this characteristic only to the point where the plate voltage increases suddenly in value. The reason for this will be seen by referring to Fig. 5 where the curve 51 represents the voltage applied to the screen grid Il. It will be seen, from this curve that because of the coupling between the screen grid circuit and the plate circuit, the screen grid voltage is reduced as the plate voltage is increased. At the point where the sudden ncrease in plate voltage occurs, there is a corre spending sudden decrease in screen grid voltage. This sudden decrease in screen grid voltage starts the accumulatve action previously described, whereby the tube is biased to cut-off almost instantaneously. The curves shown in Figs. 6 and 'I show somewhat more accurately than the curves in Figs. 2b and 2c how the plate voltage and the screen grid voltage change as the tube 5 oscillates.

In Fig. 8, the curve 59 shows the frequency of the oscillator output plotted against changes in plate voltage, While the curve 6I shows the frequency of the oscillator output plotted against changes in screen grid voltage. It will be seen that as the plate voltage is increased, the frequency is increased, assuming that the screen grid voltage remains constant and that, as the screen grid voltage is increased, the frequency is lowered, assuming that the plate voltage remains constant. Since in practice, the plate and the screen grid are supplied with voltages from the same power source, whenever there is an increase in plate voltage, there is a corresponding increase in screen grid voltage. As a result, a tendency for a frequency increase is counter-balanced by a tendency for a frequency decrease, and the frequency of the oscillator output remains substantially constant with a considerable variation in the applied voltages.

'Ihe reason for the above described frequency characteristic of the oscillator will be understood by referring to Figs. 9 and 10.

In Fig. 9, the increase in plate current plotted against time for the case of 450 volts on the plate is represented by the curve 63. A similar curve 65 is shown for the case of 470 volts on the plate. As will be understood from the previous description of the oscillator operation, Whenever the plate current reaches a value indicated by the dotted line 61, the tube is suddenly biased to cutoff. It Will be seen that this value of plate current is reached sooner with the higher voltage on the plate whereby the frequency of oscillation is increased.

Referring to Fig. 10, two plate voltage-plate current characteristic curves of a screen grid tube are shown. The curve 69 shows the characteristic of the tube when the screen grid has 80 volts applied thereto, while the curve 1| shows the characteristic when the screen grid voltage has been increased to 85 volts. As previously explained, the sudden increase in, plate voltage, which produces the plate current cut-off, occurs when the plate current increases to a value represented by the peak 13 of the curve 1|. This peak in the curve 1| occurs at a later time than does the peak 15 of the curve 69 whereby the frequency of the oscillator output is lower for the higher screen grid voltage.

In Fig. 11, there is shown an embodiment of my invention in which a single tube supplies a sav -tooth wave of current for the deecting coils of a cathode ray tube indicated at 11. The oscillator preferably comprises a Vacum tube 19 of the screen grid type having a cathode 8|, a control grid 83, a screen grid 85 and a plate 81. In one specific embodiment of my invention, a tube was employed having the same general characteristics as the type 224.

The plate 81 is supplied through the primary 89 of a transformer 9| with suitable positive voltage. In the circuit illustrated, the voltage is supplied from a suitable voltage divider 93 connected across a source of rectified and filtered current (not shown). The screen grid 85 is supplied through the secondary 95 of the transformer 9|, with a somewhat lower positive voltage, from the voltage divider 93. The screen grid 35 is connected to the plate circuit in the same phase relation as described in connection with Fig. 1.

The plate 81 is coupled through a coupling condenser 91 to the horizontal deiectingV coils 99 of the cathode ray tube 11. Preferably, the inductance of the deecting coils 99 is considerably lower than the inductance of the transformer primary 39, whereby the greater part of the current flow in the plate circuit will be through the` deflecting coils and, therefore, be useful current. With this circuit arrangement, the frequency of oscillation is determined mainly by the inductance value of the deflecting coils 99 and by the` applied voltages.

The main function of the transformer 9| is not to make the plate circuit of the tube 19 inductive, but instead merely to transfer the voltage kick which occurs in the plate circuit into the screen grid circuit for blocking the tube. It has been found advisable to connect a by-pass condenser 9| between ground and the end of the secondary 95 which is the more remote from the screen grid 85 in order to insure that the tube will oscillate without the reception of synchronizing signals. Otherwise, the plate current would rise to a high value suicient to injure the tube as soon as synchronizing signals were no longer impressed upon the tube input electrodes. Also, a by-pass condenser |99 may be providedv for the plate circuit if desired.

The synchronizing impulses are applied through a coupling condenser |03 to the control grid 89, the control grid being connected to the cathode 8| through a grid leak resistor |95. Certain typical condenser, inductance and voltage values have been indicated on the drawings, but it should be understood that they may be varied Within wide limits.

In Fig. l2, a circuit is shown which is similar to the circuit shown in Fig. 1l, the main difference being that the voltage impulses supplied by the oscillator tube 19 are applied to a power tube |91 which supplies a saw-tooth current of the proper magnitude for the cathode ray deflecting coils 99. In the two gures, like parts are indicated by the same reference numerals. As in Fig. 11, the main part of the inductance for the plate circuit of the tube 19 is reflected into the primary of the transformer 9|, this inductance being provided by the inductance coil |99. Since it is desired that negative voltage impulses be applied to the control grid of the power tube |01, as is well known in the art, for producing the saw-tooth wave of current, the control grid, together with the inductance coil |99, is coupled through a coupling condenser to the screen grid 35 of the oscillator tube. An inspection of Fig. 7 will show that in the screen grid circuit, there are negative voltage impulses of great magnitude.

In Fig. 13, there is shown a slight modification of' the circuit shown in Fig. 12. In the two gures, like parts are indicated by the same reference numerals. The main difference between the two figures is that in Fig. 13, the inductance coil |89 has a resistor ||3 connected in series therewith, whereby the voltage impulse in the screen grid circuit is given a saw-.tooth component. This is due to the fact that a saw-tooth wave of current ows through the inductance coil |99 to produce a saw-tooth voltage wave across the resistor 3, this voltage wave being impressed upon the screen grid circuit through the transformer H9. The shape of the voltage Wave applied to the input circuit of the power tube |81 is indicated at I5. As is well known, this is the Wave shape desired for producing a perfect sawtooth wave of current in the deflecting coils 99 since they cannot .be made purely inductive.

In Fig. 14, there is shown another form of single tube deflecting circuit. The circuit is the same as the one shown in Fig. 11, except that the secondary 95 of the transformer 9| is connected to the control grid 83 instead of to the screen grid 85, and except that the synchronizing impulses are applied to the screen grid 85 instead of to the control grid. In Figs. 11 and 14, like parts are indicated by the same reference numerals.

The tube I1 shown in Fig. 14 is the one commonly employed in television receiving systems for separating the picture signals from the synchronizing signals. It is biased close to the cutoff point by means of a self-biasing resistor H9 whereby positive synchronizing impulses are passed by the tube while the more negative picture signals are not passed. The plate IZI of the tube l l1 is connected to the screen grid 85 of the oscillator tube whereby the plate impedance of the screen grid tube 19 is increased each time a synchronizing impulse appears upon the input circuit of tube lll. The operation of the oscillator shown in Fig. 14, is the same as that of the oscillator shown in the other figures. It will be noted that, in this embodiment of the invention, the control grid 83 is held at a suitable negative bias by means of a self-biasing resistor |23.

My improved oscillator is pulled into step with the synchronizing signals in the same manner whether the synchronizing impulses are applied to the control grid or to the screen grid of the oscillator tube. In either case, a negative synchronizing impulse impressed upon the grid of the oscillator tube causes a sudden increase in the plate impedance of that tube whereby there is produced a voltage impulse in the plate circuit which is transferred into the screen grid circuit (or to the control grid circuit as in Fig. 14) in the proper phase relation to increase the tube impedance still more. This action is accumulative and the tube is rapidly brought to cut-off.

It has also been found that the oscillator may be synchronized by applying positive synchronizing impulses thereto but when synchronized in this manner, the synchronizing impulses must have a considerably greater magnitude than when they have a negative polarity.

From the foregoing description it will be apparent that various other modifications may be made in my invention and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and as are set forth in the appended claims.

I claim as my invention:

l. In combination, a vacuum tube including a cathode, a control grid, a screen grid, and a plate and having a screen grid circuit and a substantially purely inductive plate circuit, means for impressing a positive direct-current potential upon said screen grid, means for impressing a positive direct-current potential upon said plate, and means forso coupling said plate circuit to said screen grid circuit that said screen grid is made negative in response to said plate receiving a positive impulse thereon whereby said tube oscillates at a certain frequency, said screen grid circuit and said plate circuit being non-resonant at said frequency.

2. In combination, a screen grid tube including a control grid, a screen grid and a plate and a substantially purely inductive screen grid circuit and a substantially purely inductive plate circuit, means for applying a positive direct-current voltage to said screen grid, means for applying a positive direct-current voltage to said plate, and means for coupling said plate circuit to said screen grid circuit in such direction that said screen grid is made negative in response to the current in said Y plate circuit reaching a certain value whereby Said tube oscillates at a certain frequency, said screen grid circuit and said plate circuit being nonresonant at said certain frequency.

3. In combination, a vacuum tube including a cathode, a control grid, a screen grid, and a plate and having a substantially purely inductive plate circuit, meansV for applying a positive direct-current potential to said screen grid, means for applying a positive direct-current potential to said plate, and a transformer having a primary and a secondary, said primary being included in said plate circuit and said secondary being connected between said cathode and one of said grids in such direction as to make said one grid negative when the current in said plate circuit reaches a certain value whereby said tube oscillates at a certain frequency, said screen grid circuit and said plate circuit being non-resonant at said certain frequency.

V4. An oscillator comprising a vacuum tube of the screen grid type having a cathode, a control grid, a screen grid, and a plate, an inductance coil connected between said plate and cathode, means for applying a direct-current voltage to said plate whereby the plate current flow through said coil increases up to a certain value, means for applying a direct-current voltage to said screen grid having a value such that the voltage applied to said plate suddenly increases when said plate current reaches a certain value, and means for applying said voltage increase to said screen grid in reversed phase.

5. In combination, an electric discharge tube having a cathode, a grid, a second grid, and a plate and characterized in that secondary electrons flow from said plate when said Second grid has a certain positive voltage applied thereto greater than the voltage on said plate, said tube having a highly inductive plate circuit, a transformer coupling said plate circuit to one of said grids in such direction that said one grid is made less positive in response to said plate being made more positive whereby said tube oscillates at a certain frequency, said plate circuit and said one grid circuit being non-resonant at said certain frequency, and means for applying control voltages to the other of said grids.

6. In combination, an electric discharge tube having a cathode, a grid, a screen grid and a plate, and having a screen grid circuit and a plate circuit, means for Vapplying positive direct-current potentials to said screen grid and to said plate, a transformer having a primary winding and a secondary winding, said primary winding being connected in said plate circuit, said secondary winding beng connected in said screen grid circuit in such a direction that said screen grid is made less positive in response to said plate being made more positive, an amplifier tube having input electrodes, an inductance coil having a resistor in series therewith, means for coupling said inductance coil to one of said transformer windings whereby its inductance is reflected into said plate circuit, and means for coupling said amplifier input electrodes to s'aid grid circuit.

'7. In combination, an amplifier tube which is selectively responsive to synchronizing signals, said tube having a plate circuit, an oscillator tube having a cathode, a control grid, a screen grid, and a plate, a transformer having a primary and a secondary, said primary being connected between said plate and said cathode, said secondary being connected between said control grid and said cathode in such a direction that said control grid is made less positive in response to said plate being made more positive, and a direct current connection between said amplifier tube plate circuit and saidscreen grid whereby said screen grid is maintained positive.

ARTHUR C. STOCKER. 

